A Group III nitride compound semiconductor such as gallium nitride (GaN), which may be referred to as a “Group III nitride semiconductor” or “GaN semiconductor” in the following, has attracted attention as a material for a semiconductor device that emits blue or ultraviolet light. A blue laser diode (LD) is applied to a high-density optical disk or display, and a blue light-emitting diode (LED) is applied to a display or lighting. An ultraviolet LD is expected to be used in biotechnology, and an ultraviolet LED is expected to be used as an ultraviolet source of a fluorescent lamp.
A substrate of the Group III nitride semiconductor (e.g., GaN) for a LD or LED is formed generally by the heteroepitaxial growth of a Group III nitride single crystal on a sapphire substrate by vapor phase epitaxy. Examples of the vapor phase epitaxy include metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), and molecular beam epitaxy (MBE).
On the other hand, a method using liquid phase epitaxy instead of vapor phase epitaxy also has been studied. The equilibrium vapor pressure of nitrogen at the melting point of the Group III nitride single crystal such as GaN or AlN is 10,000 atm or more. Therefore, conventional liquid phase epitaxy of GaN has required the conditions of 8000 atm (8000×1.01325×105 Pa) at 1200° C. (1473 K). In recent years, however, it has become clear that GaN can be synthesized at a relatively low temperature and pressure, i.e., 750° C. (1023 K) and 50 atm (60×1.01325×105 Pa) by using an alkali metal (e.g., Na) as a flux.
For example, JP 2002-293696 A (Patent Document 1) discloses a method in which a mixture of Ga and Na is melted at 800° C. (1073 K) and 50 atm (60×1.01325×105 Pa) in a nitrogen gas atmosphere containing ammonia, and a single crystal is grown from this melt (material solution) for 96 hours. The single crystal thus obtained has a maximum crystal size of about 1.2 mm.
Another method also has been reported in which a GaN crystal layer is deposited on a sapphire substrate by metal organic chemical vapor deposition (MOCVD), and then a single crystal is grown by liquid phase epitaxy (LPE).
The liquid phase epitaxy of a GaN crystal using an alkali metal (e.g., Na) as a flux will be described below. FIG. 8 is a schematic view showing the configuration of a growth apparatus. The growth apparatus includes a source gas supply unit 801 for supplying a nitrogen gas (source gas), a pressure regulator 802 for regulating the pressure of a growth atmosphere, a reactor (stainless steel container) 803 for crystal growth, and a heating unit (electric furnace) 804. A crucible 805 is set in the stainless steel container 803. A connection pipe 806 through which the source gas flows from the source gas supply unit 801 to the stainless steel container 803 is made of a SUS material. The crucible 805 is made of alumina (Al2O3). The temperature of the electric furnace 804 can be controlled at 600° C. (873 K) to 1100° C. (1373 K). The ambient pressure can be controlled at 100 atm (100×1.01325×105 Pa) or less by the pressure regulator 802. In FIG. 8, reference numeral 807 denotes a stop valve, and 808 denotes a leak valve.
Na (flux) and metal gallium (material) are weighed out in a predetermined amount and placed in the crucible 805. Moreover, a substrate obtained by growing GaN on a sapphire substrate with MOCVD is put in the crucible 805 as a seed crystal. The crucible 805 is inserted in the stainless steel container 803. Then, the stainless steel container 803 is set in the electric furnace 804 and connected to the connection pipe 806, which is connected to the source gas supply unit 801. The growth temperature is 850° C. (1123 K) and the nitrogen ambient pressure is 30 atm (30×1.01325×105 Pa). A GaN single crystal is grown by maintaining the growth temperature for 30 hours and 96 hours, respectively. In a growth time of 30 hours, the GaN single crystal has a thickness of 50 μm. In a growth time of 96 hours, the GaN single crystal has a thickness of 700 μm.
This result shows that in the current growth apparatus, it takes about 20 to 30 hours to dissolve nitrogen in a Ga/Na melt (material solution) until the solution is supersaturated. The growth rate in the thickness direction (C-axis direction) is about 10 μm/hour.
However, further improvements in growth rate and quality are being demanded in the field of a Group III nitride single crystal including GaN.
Patent Document 1: JP 2002-293696 A